In an earlier U.S. Patent to Charles Cain U.S. Pat. No. 3,500,365, and in another earlier U.S. Pat. No. 4,007,454 to Charles Cain and myself, there are disclosed the use of rotating magnetic and electric fields, respectively, in a device for remotely measuring the orientation of a meter hand or other rotating object. As it turns out, the actual theory of operation is not exactly a rotating electric or magnetic field. Rather, more correctly, the field in the space adjacent the meter hand or other rotating object is such that the resultant vector representing maximum field strength rotates. However, a "rotating electric or magnetic field" is approached or approximated and hence, the terminology is not wrong. As used hereinafter the above theory of operation will then sometimes be referred to as rotating electric or magnetic field. As far as this application is concerned, the magnetic field case is directly analogous to the electric field case and leads to similar results. Therefore, for the most part, the instant application will be directed to a description of the electrode array, and the magnetic pole piece array will become apparent.
In the meter-reading device mentioned hereinabove, there is described a circular array of electrodes arranged in a plane with a detecting electrode located at the center thereof, and coplanar with the array. By application of polyphase voltage to successive electrodes in the circular array, an electric field (maximum field strength vector) is caused to effectively rotate in the adjacent air space. The detecting electrode, which is located at the center of the field's rotation, finds itself in a region of constant field strength. Consequently any measurement of its electrical potential will give a constant value as the field rotates. The array is positioned immediately adjacent and before a meter dial in such position that the detecting electrode confronts the end of the meter shaft and the surrounding electrodes confront the circular path described by the meter hand. As a result, the meter hand is immersed in the rotating field and "samples" the field and, by capacitive coupling, communicates the field strength behavior it there finds to the detecting electrode near its opposite end. The rotating field is such that the meter hand tip experiences a sinusoidal variation of field strength, and thus the detecting electrode likewise displays a sinusoidal variation of electrical potential. The electrical phase of the potential on the detecting electrode is determined by the angular orientation of the meter hand from a referenced point (usually zero), and a comparison of phase between this potential and that applied to any one of the electrodes in the circular array (the "reference" electrode) yields a measurement of the position of the meter hand.
The foregoing is a description of the device which remotely measures the orientation of meter hands or other rotating objects as described in the aforementioned patents, and therefore, it can be seen that it is desirable that the maximum field strength vector rotates uniformly. That is, the maximum field strength vector should have reasonably constant amplitude and rotate at a substantially uniform angular speed (also referred to as being linear). Otherwise the phase difference between the potentials of the detecting and reference electrodes would not be simply proportional to the hand position, but would be greater or less as the amplitude or speed varied. This would cause the device to be less sensitive in some angular regions than other, and unless compensated by field-correction means or by electronic means, could materially reduce the utility of the device. It has been found that the shape disclosed in U.S. Pat. No. 4,007,454 was not altogether satisfactory in this regard, i.e. the field strength vector did not maintain constant amplitude or rotate at uniform angular speed. Further the fact that all meter hands are not needle-like, but sometimes are tapered added to the problems in the embodiment described in earlier U.S. Pat. No. 4,007,454.